The present invention relates to a stepping motor driving circuit using static induction transistors.
Most integrated circuits for timepleces are fabricated by using C-MOS transistor technology. These C-MOS transistors are generally of the normally OFF type so that their power consumption is small. It is known in the horology art that in order to fabricate a high accuracy wristwatch having an error on the order of seconds per year, it is necessary to use a quartz oscillator having a good temperature characteristic and a capability of oscillating at several MHz frequency. Since almost all of the current consumption in C-MOS transistors is produced by flowing current therethrough at the switching operations, such transistors ae disadvantageous in that the power consumption is increased in proportion to the operating frequency. Therefore, in the case that the operating frequency of the integrated circuit is approximately several MHz, although a static induction transistor (SIT) is close to a normally ON type as compared with the C-MOS transistor, the SIT is desirable due to its lower power comsumption.
The conventional arrangement of an integrated circuit for a timepiece will be described in conjunction with FIG. 1. The current for operating an oscillator 2 is supplied from a constant current circuit 1a so as to improve the operable temperature range for the oscillator 2. The current for a pair of frequency dividing circuits (which include a waveform shaping circuit) 3a and 3b is supplied from a constant current circuit 1b. An output from the oscillator 2 is changed into drive pulses having a period of 0.5[sec] and a pulse width of about 8[ms] in the frequency driving circuits 3a and 3b. A driving circuit 4 is controlled in accordance with the drive pulses to rotate a motor M.
FIG. 2a is a sectional view illustrating the structure of a SITL device used in the oscillating circuit 2 and the frequency dividing circuit (including the waveform shaping circuit) 3. A buried layer 7 is formed by diffusing an N type impurity into a P type substrate 8, and then, an N.sup.- epitaxtial layer 6 (which has, for example, an impurity density of 3.times.10.sup.13 /cm.sup.3) is formed. The SIT includes a gate G formed by diffusing a P.sup.+ impurity into the bottom of a V-shaped concave portion 9, and a drain D formed by diffusing an N.sup.+ impurity into the region above the gate G and the burid layer 7. The SIT structure is known as a step type static induction transistor. A lateral PNP transistor (PNP Tr.sub.1) is used as a load transistor of the SIT. The load transistor is composed of a collector which also acts as the gate G, an emitter which is formed during the same fabrication step as that in which the gate G is formed and which functions as an injector Ij, and a base which acts as a source S. An insulating layer 5 is made of a silicon oxide. A wire 10 is made of an aluminum, for example. FIG. 2b is an equivalent circuit diagram of FIG. 2a and the PNP transistor Tr.sub.1 is used as a bias transistor of the SIT Tr.sub.2.
FIG. 2c illustrates a sectional structure of the lateral PNP transistor used in the constant current circuit 1, the oscillating circuit 2 and the driving circuit 4. An emitter E.sub.1 and a collector C.sub.1 are formed by the use of P type impurity on the bottom of a V-shaped concave portion 11 and are fabricated in the same step as that of the gate G. The base B.sub.1 is drawn out by a V-shaped concave portion 12 and projects from the surface as shown. FIG. 2d is a sectional view of a vertical NPN transistor (NPN Tr) used in the constant current circuit 1 and the driving circuit 4. The NPN Tr consists of an emitter E.sub.2 formed by a similar step to that for forming the drain D, a base B.sub.2 formed by diffusing a P type impurity, and a buried layer which acts as a collector C.sub.2.
The circuit composition of the driving circuit is illustrated in FIG. 3. A Tr.sub.3 and a Tr.sub.5 are PNP transistors, and a Tr.sub.4 and a Tr.sub.6 are NPN transistors. The coil of the motor M is connected between the group of the Tr.sub.3 and the Tr.sub.6 and the group of the Tr.sub.4 and the Tr.sub.5, and the transistor groups are alternately switched ON and OFF to rotate the motor. In normal operation, the motor M is rotated by 0.5 revolutions per second. Since the time required for 0.5 revolution of motor takes about 8[msec], the motor is in a non rotating condition for almost all of the one second duration. When any mechanical shock is applied to a wristwatch during the non-rotating condition (stop mode) of the motor, there is possibility that the shock will cause mis-rotation of the motor and the accuracy of the watch may thereby be lowerred due to the unwanted rotation of the motor. To eliminate such an unwanted rotation, in the conventional driving circuit 4, the Tr.sub.4 and the Tr.sub.6 are placed in a conducting condition during the duration of the stop mode of the motor and the motor is thereby placed in a damped condition. This damping method is extremely useful in order to prevent the motor M from mis-rotating by an externally applied mechanical shock.
However, since the Tr.sub.4 and the Tr.sub.6 are bi-polar transistors, the base current flows even when the motor M is in a stopped, i.e., non-rotating, condition, so that, current comsumption occurs in the driving circuit 4.